Hydraulic fracturing as utilized herein refers to a process whereby a fluid is pumped into a well bore communicating with a subterranean reservoir under sufficient pressure to fracture the matrix of the subterranean geological formation. As these pressure forces increase, they commence and propagate fractures (fissures or cracks) in the reservoir matrix. The dimensions of the fractures generally increase by continuing to pump the pressurized fluid into the formation through the well bore.
Proppant is typically added to prop the fractures open and facilitate oil and gas recovery from the well after the treatment has been completed. The materials generally comprise of treated sands and man-made ceramics with specific gravities of 2.5 to 3.5. In order for a fracturing treatment to be effective and allow the flow of oil and gas from a reservoir to a well, proppant must be distributed in a manner that is conducive to improving the conductivity between the payzone and the wellbore.
Conventional hydraulic fracturing fluids consist of water or water-based fluids utilizing thickening agents or gels to aid in proppant transport so that it can sufficiently transport the proppant within the fracture. For low viscosity fluids like CO2, proppant transport is limited, in one respect, due to the lack of additive systems available that may enhance viscosity and improve the proppant carrying capacity of the fluid.
Utilization of liquid carbon dioxide (LCO2) in fracture treatment of oil and gas formations is well known and has certain advantages in water sensitive and low pressure formations. First, the use of LCO2 enables a significant reduction in water volume utilized, which minimizes formation damage caused by the water and second, it promotes water flow-back (i.e., retrieval of water introduced, or produced, in the fracture treatment) through expansion when pressure is let off the fractured formation. However, with a predominantly LCO2 fracturing fluid, a major drawback exists because of the less than optimal proppant transport characteristics of this fluid resulting in poor placement of proppant.
Ultra-light weight proppants (ULWP) materials have been developed to combat transport issues in thin fluids through a reduced material density. These proppants generally have specific gravities ranging from ˜1 to 2. Proppants with specific gravity close to 1 are especially useful as their transport mimics that of the carrier fracturing fluid. These proppants, however, are undetectable by a nuclear densitometer, a common device used in fracturing operations to detect proppant concentrations. A difference in density between the proppant and the carrier fluid is required in order for the proppant concentration to be measured by the nuclear densitometer.
For example, U.S. Pat. Nos. 3,657,532 and 4,618,939 disclose the use of nuclear densitometers for non-intrusive and continuous measurement of densities of flowing fluids or slurries. Typically, these instruments include a gamma radiation source. The radiation is high frequency, high energy, and exhibits a high penetration depth with a wavelength on the order of 10−12 meters (smaller than the diameter of an atom). This allows emitted photons to pass through treatment piping and reach the detector. Gamma ray attenuation (by absorption and scattering) occurs at the atomic and subatomic level. As a result, gamma attenuation is dependent on:                atomic number and density of material it is passing through, & the        thickness of the material.        
The attenuation of gamma rays is proportional to the density material. Meaning materials having a relatively high density (e.g. lead, bismuth, and tungsten) are able to absorb gamma radiation more easily than materials with a lower density (e.g. aluminum, plastics). For treatment fluids containing proppant, the detection of solids is made possible by the proppants density being of 2 to 4 times greater than that of the carrier fluid. Consequently, for proppant materials with a density close to that of the carrier fluid such as ULWP (i.e., specific gravity of close to 1) detection by a nuclear densitometer is not possible.
For control of proppant concentration in the fracturing treatment fluid stream, generally the densitometer is used as a feedback device for a control loop where corrections are made to the proppant flow rate by the control system in order achieve a setpoint concentration. In the control system described in U.S. Pat. No. 4,779,186, a densitometer is employed to adjust the speed of a screw conveyor regulating the proppant delivery rate into the base treatment fluid.
U.S. Patent Application Publication No. 2015/0060065 A1 describes a control system, associated methodology, and apparatus for implementation of an eductor-mixer technique to provide the capability to inject and meter proppant material into a non-aqueous fracturing fluid stream. The system utilizes a solids-conveying liquid eductor instead of a conventional auger to mix and accelerate proppant within the main fracturing liquid stream. The control system utilizes at least one valve for controlling the flow of proppant from one or more pressurized proppant reservoir into the eductor; thereby mixing the material with the motive stream. Gas and/or liquid is fed to the top of the proppant reservoir to control the pressure inside the proppant reservoir. Modifying the pressure inside the proppant reservoir extends the range of achievable proppant flow rates from the reservoir into the eductor.
Direct and accurate measurement of proppant concentration is quite important for proper control of the system described above. Therefore, the densitometer is a crucial instrument during operation. With ultra-light weight proppant (ULWP), measurement by this important instrument is negated by the relatively similar density to the carrier fluid, leaving accurate concentration control impossible.
To overcome the deficiencies in the related art, the present invention provides the application of a non-nuclear optical device using near-infrared (NIR), visible (Vis), or ultraviolet (UV) spectral ranges for the specific use in the detection and measurement of proppant blended into a fracturing fluid stream and thereby used as a means to regulate proppant concentration through its integration into a control system. The NIR/Vis/UV-based device addresses specific needs in the measurement of ultra-light weight proppant (with densities close to a specific gravity of 1), which is undetectable by conventional nuclear densitometers. The eliminated compliance and associated regulatory cost required for operating a nuclear device is also a benefit that can be applied to more typical sand proppant applications (i.e. proppants with a specific gravity greater than ˜2).
Although NIR/Vis/UV based devices have been utilized in the oilfield, the focus has been on property monitoring for chemical characteristics and movement of fluid. In U.S. Patent Application Publication Nos. 2010/022435, 2014/061449, and 2015/015884 fluid interactions with NIR, visible, and ultraviolet radiation is used to monitor molecular characteristics rather than proppant concentration control. For example, U.S. Patent Application Publication No. 2011/0214488 describes the use of NIR for detection of fluorescent nanoparticle-based tracer. While the use of a spectrometer is used in detection tracking of fracturing fluid movement, the art is non-related in application to proppant concentration control.